An automatic vehicle identification (AVI) system uses RF signals to communicate information between a vehicle tag and a reader apparatus. AVI tags generally are powered by a compact and inexpensive battery and may be one of two types: "Read-Only" tags allow the reader apparatus to read information previously stored in the tag's memory, while "Read/Write" tags can also write new information into the tag's memory in response to signals from the reader. Read/Write tags are more complex than Read-Only tags and use more current during the active part of a transaction. The active state current required by the Read/Write tag usually is so high that it is impractical to power the tag continuously in its fully active state. Accordingly, it is common practice to maintain the tag in a low-current background state between transactions and activate it in response to an RF activation signal from a reader apparatus only when needed for a transaction.
The reader apparatus senses changes in RF reflectivity that result from the coded backscatter modulation signal that the tag emits. Some Read/Write tags emit this backscatter modulation signal--i.e., they continue to function in the "read" mode--while the tag logic remains in the low-current background state. For "write" functions, the Read/Write tag also must have a means for receiving a modulated RF "write data" signal from the reader and converting it by detection and amplification into a form that can be used by the tag's logic circuitry. Most tags, being aimed at low cost, use simple "direct detection" of ASK (Amplitude Shift Key) or "ON/OFF" RF signals to form a low voltage signal pulse train, whose amplitude depends on the strength of the received RF signal and is therefore of variable amplitude. This pulsed signal has to be amplified to the tag circuit's "logic level"--usually at whatever battery voltage is used in the tag, e.g., 3 volts for a lithium cell. Implementing this function is made difficult by the fact that the detected signal amplitude might vary from millivolts to volts, depending on the strength of the reader RF signal received at the tag.
In addition, tag RF detection circuits must be able to distinguish stray RF signals to prevent the tag from being activated inadvertently and to insure that unwanted data is not passed to the tag. Some existing tag systems use coded RF signals to overcome this problem. In general, however, building selectivity into the tag's RF detection circuitry increases power consumption and makes battery operation more difficult and expensive.
While there are well known means for detecting RF signals, the excessive power consumption of most such circuits makes them impractical for inclusion in a battery-operated tag. An earlier Amtech tag addressed the power consumption problem by using a high-gain bipolar transistor for threshold RF detection and signal amplification. The bipolar NPN transistor conducts at about 550 mV, yet passes almost no current until the detected RF voltage level rises to about 450 mV, making it well suited for systems with a high RF threshold level--e.g., 550 mV. However, that circuit is vulnerable to background interference from both continuous wave (cw) and modulated RF sources, and will not work in systems using low-level reader-transmitted RF signals.
Due to concerns about health risks associated with long-term exposure to RF energy, new tag systems are being designed to require very low RF threshold levels, e.g., 5mV-50 mV, requiring greater sensitivity in the tag's RF threshold detection circuit. Threshold detection of dc voltages in this range is difficult to do economically because conventional differential-amplifier input voltage comparators have an offset voltage of up to .+-.15 mV due to random variations in semiconductor processing, and that offset voltage tends to vary with ambient temperature. The offset voltage shifts the threshold level and sets a lower practical limit on the RF level that can be detected reliably. While there are known techniques for trimming out the offset voltage, they are expensive to implement and may not perform consistently over changing temperatures and supply voltage levels.
Additionally, the RF level received at the tag can range from 5 mV up to 1V or more, creating a risk of overload and distortion at higher signal levels. Variation in the received RF signal also makes amplifying the read/write signal to the logic level of the tag circuitry more problematic. Therefore, what is needed is a low level RF detection circuit that consumes little power and is not susceptible to background RF noise or fluctuations in the RF energy level.